Several different types of dot-matrix document printers are in common use in the prior art, including apparatus to print names and addresses on envelopes as the envelopes pass adjacent a series of printing heads. A primary purpose of such apparatus is to transport the document through the printing station at high speeds to increase the rate of production and efficiency of the operation. One such document printer is generally referred to as an ink jet printer, wherein particles of ink are deposited on the surface of the document by a series of jets formed in a matrix at the printing station, whereby the pattern developed by the matrix to form each letter is controlled electronically by a microprocessor, or other similar device. The complexity and cost of ink jet printing systems makes them prohibitive for use by smaller business operations with minimal or moderate printing demands.
Another type of dot-matrix printing system which is more compatible with smaller business operations having less printing requirements utilizes a series of staggered dot matrix print heads past which a series of documents to be printed are constantly moving at relatively high speed. Each printing head consists of a series of linearly-disposed matrix wires which sequentially impact an ink ribbon and a document passing the print head pursuant to a pre-set program to form letters on the document. The print heads are controlled by a character generator program which generates letters upon contact with the moving document. Certain of such print heads employ solenoids with moving cores attached to the matrix wires, while other systems use electromagnets to activate small hammers that ballistically propel the print wires towards the inked ribbon and then into contact with the document to be printed. An example of the latter described dot print head is marketed by Lear Siegler under the name Ballistic (TM) Matrix Print Head, and is illustrated in conjunction with the preferred embodiment of the present invention. However, it is to be understood that other matrix print heads can be utilized in the disclosed apparatus without varying from the scope of the inventive concepts embodied therein.
Each matrix print head utilized in the present invention comprises a series of wires disposed linearly adjacent the printing end of the head. The other end of each wire extends to one of a series of solenoid coils which are independently actuated responsive to electrical impulses transmitted by a pre-set signal generator, such as a microprocessor. In accordance with the pre-set printing program, a series of impulses are transmitted to selected solenoids, and the wires associated with each activated solenoid are propelled forward. The tip of each wire impacts an inked ribbon before contacting the document surface to be printed. The document moves constantly past the print head, and the sequence and number of wires impacting the document through the inked ribbon forms the letters and characters on the document surface.
As the wires in the print matrix head are propelled forward under the influence of the solenoids, a back-up element must be provided adjacent the side of the document opposite the printed side to arrest the forward movement of the pins. Without such back-up, the pins would be prone to pierce the ribbon and the document upon impact. Certain prior devices have utilized a flat bar opposite the print heads, which receives the impact of the print head wires. However, this means of providing a back-up for the print head wires introduces an inefficient factor into the operation of the printing apparatus. As mentioned previously, the printing device of the present invention is designed to rapidly print an address or other indicia on a document, where the document is constantly moving at high speed along a path past the print heads. The document slides across the flat back-up bar of certain of the prior devices, whereby friction tends to slow down the movement of the document.
The matrix print heads are pre-programmed to create letters and characters on a document which is moving at a pre-selected, uniform rate consistent with printing speed past the print heads. If the document speed is altered as it passes the print heads, the address can possibly be applied non-uniformly to the document, whereby one letter or character may be formed on top of another. If one side of the document must pass over a stationary bar, the friction between the bar and the document may cause the document to pass the print heads at an intermittent rather than uniform speed, causing uneven application of printed material on the document.
In addition, certain prior dot matrix printing apparatus of the type disclosed herein utilize an inked ribbon which directly contacts a broad portion of the document to be printed. The ink tends to smear or streak the document adjacent the print head since the ribbon is sequentially and repeatedly moved toward the document by the print head wires.
The above-noted defects have materially reduced the efficiency, workability, and commercial acceptance of certain prior dot matrix print systems. An object of the present invention, therefore, is to provide a novel roller platen back-up system for ballistic dot matrix printers which provide the requisite back-up for the wire print heads while at the same time providing an entry gap for moving the document past the print heads with the least amount of obstruction.
A further object of the present invention is to provide a roller platen back-up system for a dot-matrix printer which will not impede or otherwise interfere with the rapid and constant movement of a document through the printing apparatus.
Yet another object of the present invention is to provide a back-up system for a dot matrix printer which can handle documents of intermixed thicknesses without the necessity of making adjustments to the back-up system.
Yet another object of the present invention is to provide a mask or guard between the inked ribbon and document to be printed by a dot matrix printer to prevent streaking or smearing of the document during the printing operation.
These and other objects and features accomplished by the present invention will be better understood with reference to the following summary of the invention, drawings, and detailed description thereof.